1. Technical Field
The present invention relates to an arrangement for determining a maximum allowed velocity of a vehicle when going downhill when braking capabilities are considered. More specifically, the invention relates to a method for determining a maximum allowed velocity of a vehicle taking into account the inclination of the vehicle and at least one other aspect of the vehicle's braking capability.
2. Background Art
The transportation of heavy loads, such as that often performed commercially by contractors, can and is frequently performed using vehicles of the articulated hauler type. These types of vehicles may be chosen because of their capabilities to operate with large and heavy loads in areas where there are no roads. Example settings for such implementation include load transport in connection with road and tunnel building, sand pits, mines and similar environments.
These types of articulated haulers are typically constructed with a forward vehicle section, often referred to as the engine unit and including a forward frame that supports an engine and a front wheel axle. Further, haulers of this type normally have a rear vehicle section taking the form of a load-carrying unit including a rear frame that supports two wheel axles, a forward bogie axis and a rear bogie axis. The vehicle is commonly designed for switching between operations using various driving wheel combinations, all six wheels on the three axes being selectively operable as driving wheels, depending on the prevailing operating conditions. An articulated hauler may have a weight in the order of 15–30 metric tons and may be loaded with a payload in the order of 20–35 tons.
In a typical articulated hauler, the frame of the engine unit is connected to the frame of the load-carrying unit by means of a special articulation joint. This joint allows the engine unit and the load-carrying unit to pivot in relation to each other about an imaginary longitudinal axis essentially extending in the longitudinal direction of the vehicle. This articulation joint also allows pivoting about a vertical axis for steering of the vehicle. In this manner, the engine unit and the load-carrying unit are allowed to move substantially independently of each other. This reduces the stress loads acting on the vehicle, especially when operating in difficult or complex terrain.
Normally, an articulated hauler is equipped with a diesel engine and an automatic transmission having, for example, six forward gear ratios and two reverse gear ratios. For braking of the vehicle, an operating brake system is used, preferably of the hydraulic type and divided into two circuits; one circuit intended for the engine unit and another circuit intended for the load-carrying unit. The brake system includes conventional disc brakes arranged for braking the respective wheels. With respect to the transmission, there is normally an integrated retarder; that is, an hydraulic brake device that acts on a turbine shaft of the transmission for braking the vehicle.
During braking of the hauler, the driver uses a brake pedal designed in such a way that when depressed, it initially causes activation of a retarder. When depressed further, the mechanical or operating brakes are also activated and caused to operate in cooperation with the retarder. The reason for first activating the retarder is to help control the high thermal loads affecting the operating brake during mechanical braking. Thus, by primarily using the retarder, or at least initially using the retarder, reduced wear of the operating brake system is achieved and an increased operator braking comfort is also realized.
Such haulers are also often suitably equipped with a separate retarder pedal that, when depressed, will only cause braking by means of the retarder.
Further, the hauler normally includes an additional brake function in the form of a motor-brake that is a typically occurring braking function of diesel engines. A motor-brake is controlled by means of a separate control and includes a throttle that, when actuated, will restrict the flow of exhaust gases from the engine during its exhaust phase. This creates a back pressure in the exhaust system that in turn causes a braking effect.
In the case of articulated haulers, it is a general demand that the machine be able to be run at as high a velocity as possible. The choice of velocity, however, has to be made considering expected costs that may occur as a consequence of service and possible repairs required if too high a velocity is chosen.
In order to limit the velocity of an operating hauler, the driver can brake entirely with the operating brake. This way of braking however, presents a problem because the ordinary disc brakes of the operating or mechanical brake system is not designed to be continuously used for extended periods of time. This can particular be the case when braking occurs while the vehicle is traveling downhill while burdened with a heavy load. In such cases, the mechanical brakes may become overloaded, which in turn can result in a reduced braking ability and/or an unnecessary increase in brake-component wear.
To spare the operating brake, the retarder or motor-brake should be predominantly utilized. An existing problem, however, is that drivers do not always pay attention to directions given regarding which kind of brake that is the correct at different situations. Instead, they use the operating brake too frequently. Furthermore, there is a risk that drivers will not pay attention to the instructions and directions regarding braking procedures, and also run the vehicle at too fast of speeds and use the operating brake too much which compounds the detrimental effects that are suffered by the vehicle.
The above problem is especially serious when the hauler is traveling downhill with a heavy load. In such situations, it is possible for the velocity to become too high, while there is also a risk that the retarder and the exhaust gas brake will not be capable of delivering sufficient brake power to bring such excessive speed under control. From an alternative perspective, it may be considered that the retarder and the motor-brake have insufficient cooling capabilities regarding the overall braking functions. In any case, because the operating brake must also be used to reduce the machine's velocity, a risk for excessive wear and damage to the operating brake system exist.